(1) Field of the Invention
The present invention relates to a covering metal structure for a metallized metal layer in an electronic part. More particularly, the invention relates to a covering structure for a metallized metal layer in an electronic part such as a semiconductor package or a multi-layer circuit board. According to the present invention, appearance of strains or blisters on this covering metal layer is prevented, and the appearance characteristics, electric characteristics and durability of the electronic part can be improved.
(2) Description of the Prior Art
A metallized metal layer in an electronic part, which has to be prepared by non-electrode plating, such as a die attach metal layer for attachment of a semiconductor element of a plug-in type semiconductor package, a sealing metal layer for soldering of a lid member or a wire-bonding portion for attachment of a wire for connecting a semiconductor element to a lead line, has a covering structure as shown in FIG. 2. More specifically, according to the conventional technique, a metallized metal layer 2 formed on the surface of an insulating substrate 1 composed of an electrically insulating material such as ceramic or glass is covered with an intermediate metal layer 3 composed of a nickel/phosphorus alloy or a nickel/boron alloy for maintaining a sufficient bonding strength to a solder, and the surface of the intermediate layer 3 is covered with a surface metal layer 4 composed of gold for preventing degradation of the wettability of the intermediate metal layer 3 with the solder by oxidation of the intermediate metal layer 3 and also for tightly bonding a bonding wire.
Incidentally, the metallized metal layer 2 is formed from a powder of a high-melting-point metal such as tungsten (W), molybdenum (Mo) or molybdenum/manganese (Mo/Mn).
In this covering structure for a metallized metal layer of an electronic part, if a nickel/phosphorus alloy is formed as the intermediate metal layer 3 on the metallized metal layer 2 by non-electrode plating, since the surface of the metallized metal layer 2 is coarse and the phosphorus component in the nickel/phosphorus alloy is inactive, it is impossible to deposit the intermediate metal layer 3 in a uniform thickness on the entire surface of the metallized metal layer 2, and many pinholes (small holes) or voids (small gaps) are formed in the intermediate metal layer 3. If the plating solution is left in these pinholes or voids, the plating solution is caused to flow onto the surface metal layer 4 by heat applied at the step of attaching a semiconductor or lid member, with the result that spot stains are formed to degrade the surface characteristics of the electronic part. Furthermore, corrosion of the metal is advanced from these stains to degrade the electric characteristics of the part or shorten the life of the part. This is one of fatal defects of the conventional techniques.
If a nickel/boron alloy is deposited as the intermediate metal layer 3 by plating, since the adhesion of the boron component in the nickel/boron alloy to gold is poor and the thermal expansion coefficient of the nickel/boron alloy is greatly different from that of gold, the surface metal layer 4 is peeled from the intermediate metal layer 3 when heat is applied to the intermediate metal layer 3 and the surface metal layer 4 at the step of attaching a semiconductor element or a lid member, with the result that blistering is caused and it is impossible to tightly attach a semiconductor element or a bonding wire to the metallized metal layer 2. Furthermore, the mechanical strength is reduced in the blistered portion. This is another fatal defect of the conventional technique.
Accordingly, in the conventional covering structure for a metallized metal layer, which comprises a nickel/phosphorus alloy or a nickel/boron alloy as the intermediate layer, the manufacturing yield is extremely low, and increase of the manufacturing yield is eagerly desired.